Implement command to reset the pid lock

[XA21X]

Since #93, I don't believe pid is ever reset as a fail-safe?

Logs indicated that pre-generate was always returning command is already running so I was getting suspicious. I finally checked the appconfig and noticed that pid == 1, which will never not be running.

I believe this could happen if the preview generator crashes or is interrupted while executing php occ directly (via docker run/exec). If this is the case, some time-based check might still be valuable in addition to the pid check - that is, periodically update a watchdog timestamp in the appconfig and treat the previous instance as dead if it hasn't been updated for a while?

[XA21X]

Or it might be easier to just special-case PID1? :P

[joshtrichards]

I believe this could happen if the preview generator crashes or is interrupted while executing php occ directly (via docker run/exec).

Agreed, this could happen under some circumstances:

previewgenerator/lib/Command/PreGenerate.php

Lines 105 to 115 in bf04c0a

	$this->setPID();
	// Set timestamp output
	$formatter = new TimestampFormatter($this->config, $output->getFormatter());
	$output->setFormatter($formatter);
	$this->output = $output;
	$this->sizes = SizeHelper::calculateSizes($this->config);
	$this->startProcessing();
	$this->clearPID();

Or it might be easier to just special-case PID1? :P

True, it is probably most likely to happen with 1. Tempting! :)

Maybe also with Docker environments with their own PID namespaces?

If this is the case, some time-based check might still be valuable in addition to the pid check - that is, periodically update a watchdog timestamp in the appconfig and treat the previous instance as dead if it hasn't been updated for a while?

Maybe. Or just check for the other matching running processes (that look like other copies of us) outright and error out as already running in that case.

Related: #328

[st3iny]

That is weird. Running exec should not start the command with PID 1 inside the running container.

But yeah, having a command to reset the pid lock or a fail-safe is probably a good idea.

[st3iny]

In the meantime, the log can be reset by running the following raw SQL query:

DELETE FROM
  oc_appconfig
WHERE
  appid = 'previewgenerator'
  AND configkey = 'pid';

[st3iny]

The pid lock is gone now.

Ref #576